In a magnetic-field-angle measurement apparatus which measures the angle of magnetic field (magnetic-field angle) or a rotation-angle measurement apparatus which measures the rotation angle of a rotation unit by measuring the magnetic-field angle of a magnet fixed to the rotation unit, the measurement characteristics depend on a sensor element which detects the angle of the magnetic field. As such a magnetic-field-angle measurement apparatus or rotation-angle measurement apparatus, devices using a magnetoresistance element (MR element) have been known in the related art.
An anisotropic magnetoresistance element (AMR element), a giant magnetoresistance element (GMR element), a tunneling magnetoresistance element (TMR element), and the like are known as MR elements. In these elements, the resistance value of each element changes if the angle of external magnetic field (magnetic-field angle) or the external magnetic field strength changes.
As measurement performance of the magnetic-field-angle measurement apparatus, the S/N ratio (signal-to-noise ratio) is an important figure of merit.
The S/N ratio of the MR element is dominated by the amount of change in the resistance of element to changes in the magnetic-field angle. The S/N ratio is improved as the rate of change of the value of the resistance of element to a fixed magnetic-field angle change increases. The maximum rate of change of the resistance value due to changes in magnetic-field angle is called an MR ratio. The MR ratio is about 2% in the AMR element and about 10% in the GMR element, while the MR ratio reaches 50% or more in the TMR element.
In particular, the MR ratio of the TMR element has exceeded 100% due to improvements or the like in a tunnel-insulator layer used in the element in recent years, and a TMR element with the MR ratio of even 600% has also been reported.
As a sensor which detects the rotation angle of the rotation unit using a TMR element, for example, a magnetic encoder disclosed in PTL 1 is known.
The present invention relates to a magnetic-field-angle measurement apparatus and a rotation-angle measurement apparatus which use a TMR element. In particular, the present invention relates to a magnetic-field-angle measurement apparatus using a TMR element with a high MR ratio and a rotation-angle measurement apparatus using the same.
As a form of GMR element, there is a granular GMR element formed by dispersing paramagnetic particles with a diameter of about 5 nm in an insulating material. Electrons tunnel among the paramagnetic particles, so that current flows through the granular GMR element. If the external magnetic field changes, the angle of magnetic field in paramagnetic particles also changes, and the resistance value of the layer changes accordingly. Since the operation of the granular GMR element is based on the tunneling phenomena between particles, the granular GMR element is sometimes regarded as a tunneling magnetoresistance element. However, the MR ratio is only about 10%. In addition, since the granular GMR element is an element that does not have a pinned magnetic layer, it is not covered in the present invention.
FIG. 21 shows a structure of a TMR element having a pinned magnetic layer. The TMR element 51 has a layered structure in which a tunnel-insulator layer 12 is interposed between a pinned magnetic layer (pinned layer) 13 and a free magnetic layer (free layer) 11. The pinned magnetic layer 13 and the free magnetic layer 11 are formed of a magnetic material having components of Co, Fe, Ni, and the like, for example. The tunnel-insulator layer 12 is formed of an insulator, such as aluminum oxide (Al2O3) or magnesium oxide (MgO), and the film thickness is about 0.5 to 2 nm.
The angle of magnetization 22 in the pinned magnetic layer 13 is fixed in a predetermined direction at the angle θp, and is not changed by the external magnetic field 30. On the other hand, the angle of magnetization 20 in the free magnetic layer 11 changes at the angle θf according to the angle θm of the angle of external magnetic field 30.
In the TMR element, the resistance value between the pinned magnetic layer 13 and the free magnetic layer 11 changes depending on the angular difference Δθ=θf−θp between the angles of magnetization. The resistance value of the TMR element is a minimum value R(P) when the angular difference Δθ between the angles of magnetization of the free magnetic layer 11 and the pinned magnetic layer 13 is 0 (Parallel) and is a maximum value R(AP) when the angular difference Δθ between the angles of magnetization is 180° (Anti-Parallel). The MR ratio (MR) of the TMR element is defined by the following Expression.
                    MR        =                                                            R                ⁡                                  (                  AP                  )                                            -                              R                ⁡                                  (                  P                  )                                                                    R              ⁡                              (                P                )                                              =                                                    R                ⁡                                  (                  AP                  )                                                            R                ⁡                                  (                  P                  )                                                      -            1                                              [                  Expression          ⁢                                          ⁢          1                ]            
In the related art disclosed in PTL 1, a rotating magnetic unit in which N and S poles of a magnet are alternately disposed is provided in a rotation unit to be measured, and the angle of the magnetic field which reverses according to the rotation of the rotation unit is detected by a TMR element. In this case, the TMR elements are arranged in a bridge configuration and used as a magnetic encoder.
FIG. 22 shows a bridge configuration 60 including four TMR elements. Magnets are disposed with repetition interval of λ in the rotation unit which is an object to be detected that is in the rotation state. Corresponding to this, TMR elements 51a, 51b, 51c, and 51d are provided with interval of λ/4. Accordingly, since the TMR elements 51a and 51c are provided with interval of λ/2, the angle of the magnetic field at the position of the TMR element 51c is θ=θ0+180° when the angle of the magnetic field at the position of the TMR element 51a is θ=θ0. Therefore, as described above, when the resistance value of the TMR element at the excitation voltage e0 side is a maximum value in the bridge configuration shown in FIG. 22, the TMR element at the ground (GND) side of 0 V has a minimum resistance value. In this manner, at the voltage between signal terminals (V2−V1), a signal change according to the rotation state of the rotation unit is observed.
In order to improve the measurement accuracy of the rotation angle of the rotation unit in the method disclosed in PTL 1, it is necessary to narrow the repetition interval of magnetization by increasing the number of poles of magnetization of the rotation unit so that the angle of the magnetic field is reversed even when the rotation angle changes slightly. In this case, however, the manufacturing cost increases.
In contrast, there is a method of measuring the magnetic-field angle, which changes in a range of θ=0 to 360°, without increasing the number of poles of magnetization of a magnet when measuring the rotation angle of the rotation unit. In this case, however, it is difficult to measure the magnetic-field angle accurately. In particular, the inventor has found a problem in that a decrease in measurement accuracy is more noticeable when a TMR element with a large MR ratio exceeding 100% is used.